1. Field of the Invention
The present invention relates to a lithographic apparatus and in particular to a method for testing a lithographic apparatus to measure and/or analyze, and to data processing apparatuses and computer program products for implementing parts of such a test method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it and critical line width in a developed metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
In order to better control scanner functionality, a module has been recently devised which automatically drives the system towards a pre-defined baseline each day. This scanner stability module retrieves standard measurements taken from a monitor wafer using a metrology tool. The monitor wafer had been previously exposed using a special reticle containing special scatterometry marks. Using the monitor wafer and that day's measurements (and possibly the historical measurement data from the previous days), the scanner stability module determines how far the system has drifted from its baseline, and then calculates wafer-level overlay and focus correction sets. The baseline can be defined either directly by the reference layer on the monitor wafers (in this case baseliner will drive the system towards minimal overlay on the baseliner monitor wafers) or indirectly by a combination of the reference layer on the wafers and the target overlay fingerprint (in this case baseliner will drive the system towards defined target overlay fingerprint on the baseliner monitor wafers). The lithography system then converts these correction sets into specific corrections for each exposure on subsequent production wafers.
While known systems provide correction of patterning performance characteristics between different apparatuses and between different parts of a substrate within an apparatus, the known techniques do not address variation within the scanning period in a scanner-type lithographic apparatus. Due to dynamic effects such as radiation heating of the patterning device, a projection lens or the substrate, distortion of the image can actually vary during the course of a scan. Although these variations are systematic and therefore predictable at least to some extent, current systems must simply accommodate this variation within their average performance criteria.